strverscmp: migrate from "strverscmp.h" to <string.h>

strverscmp: migrate from "strverscmp.h" to <string.h>

* modules/string (Makefile.am): Add new hooks.
* modules/strverscmp (Files): Remove strverscmp.h.
(Depends-on): Add string.
(configure.ac): Add indicator.
(Include): Mention new header.
* m4/string_h.m4 (gl_HEADER_STRING_H_DEFAULTS): Provide new
defaults.
* m4/strverscmp.m4 (gl_FUNC_STRVERSCMP): Inform string module of
results.
* lib/strverscmp.h: Delete.
* lib/string.in.h (strverscmp): Provide declaration, when needed.
* tests/test-strverscmp.c (includes): Adjust client.
* lib/check-version.c (includes): Likewise.
* NEWS: Document the change.

Signed-off-by: Eric Blake <ebb9@byu.net>